The present invention relates to a thermoplastic polyester composition which is particularly useful as a raw material for a powdered paint.
Powder coating has advantages in that no solvent is required to result in less environmental pollution, that a coating material can be recovered and re-used and, that a coating film having a thickness of 50 to 700 .mu.m can be formed in one coating step and is now acquiring wide acceptances. A thermoplastic polyester resin, however, has hardly been put to practical use as a powdered paint in spite of its high adhesion and excellent weathering resistance. The present invention provides a thermoplastic polyester resin composition which can afford a coating film having a high adhesion, a good state of surface and a high impact resistance under mild coating conditions.
Up to this time, a thermosetting polyester has been used as a raw material for a powdered paint mainly together with an epoxy, isocyanate or melamine compound. On the other hand, it is also known that a thermoplastic polyester resin is useful as a material for fluidized bed dip coating or electrostatic powder coating. Known processes include, for example, a process wherein a mixture of a polyester selected from among poly(neopentyl terephthalate), copolyester of 1,4-dihydroxymethylcyclohexane with terephthalic and isophthalic acids and a celullose compound such as acetylcellulose is used as described in Japanese Patent Laid-Open No. 97023/1974, a process wherein polyethylene iso/terephthalate containing isophthalic acid component in an amount of 8 to 20 molar % is used as described in Japanese Patent Laid-Open No. 41367/1984, and a process wherein polyester derived from 1,4-butanediol and terephthalic acid and having a specified composition and specified physical properties is used as described in Japanese Patent Laid-Open No. 56424/1975.
These known processes aim at overcoming a problem that a thermoplastic polyester resin exhibits a poor adhesion to a substrate, causes cracks when rapidly cooling or bending and gives a poorly lustrous surface in spite of its excellent characteristics as a coating material such as weathering resistance.
Thus, the process described in the Japanese Patent Laid-Open No. 97023/1974 relates to an improvement in gloss of the surface of a coating film which comprises adding a large amount of a cellulose compound to a polyester having a specified composition. The process described in the Japanese Patent Laid-Open No. 56424/1984 relates to an improvement in adhesion to a substrate and in bending processability which comprises using a copolyester obtained by copolymerizing isophthalic acid with polyethylene terephthalate in a specified ratio.
However, these improvements cannot give a sufficient effect to be used as a powdered paint. Especially the adhesion, impact resistance and the like thus improved are still far from levels for practical use.
Furthermore, the process described in the Japanese Patent Laid-Open No. 56424/1975 relates to an improvement in the above disadvantages which comprises specifying the composition of monomers and the physical properties.
Although the above known methods have improved various disadvantages, they have still disadvantages in that the impact resistance, particularly the thermal shock resistance in a heating-colling cycle is low and that the substrate to be coated by the fluidized bed dip coating method must be heated to 250.degree. C. or above. Consequently, they must be further improved to be put to practical use.